In the last two decades there has been a growing interest
towards the nanoworld. The scientific community was curious to cast
new light on the structure of organic, inorganic and biological materials,
probing their chemical and physical properties on a molecular scale
and comparing the properties of a single molecule with those of an
ensemble or Avogadro number of molecules. Manipulating single molecules
at room temperature, stimulating and visualizing in-situ chemical
reactions at surfaces are just few examples of how the scientific
community was able to approach to the nanoworld. In 1982 the Scanning
Tunneling Microscope (STM) was invented. This technique enables to
generate real-space images of surfaces with a nanometer scale resolution.
This discovery represented also a big improvement for the development
of miniaturised electronic devices. Even greater importance had the
invention of the Atomic Force Microscope, also called Scanning Force
Microscope (SFM), that allowed to extend investigations to insulating
materials, like polymers and biomolecules.

On the other hand, the report in 1977 about the high
electrical conductivity of a p-conjugated
molecule, namely trans-poly(acetylene), that can be achieved
upon p and n-doping opened new avenues of exploration
for chemistry, physics and technology.

Both these two discoveries were recently awarded with
the Nobel prize. Based on these two developments, the aim of this
thesis was to grow highly ordered molecular nanostructures of conjugated
(macro)molecules with well defined chemical functionalities and physical
properties that arise from both the molecules themselves and their
order at a supramolecular level. These architectures can be useful
for building molecular based electronic devices, in particular molecular
nanowires. Scanning Probe Microscopies were essential for this project
because they allowed to investigate with a sub-molecular resolution
the structures and the dynamics of (macro)molecular architectures
self-assembled at surfaces.

Chemisorption and physisorption were used alternatively
to design (macro)molecular nanostructures from p-conjugated
systems. The accomplishment of these two different approaches were
performed in the frame of extensive collaborations with of Professors
Jean - Marie Lehn and Klaus MŘllen and their co-workers who were responsible
for the synthesis of the molecular systems.

In the first case, Self-Assembled Monolayers of thiol-end
functionalized alkanes and alkenes were grown both on
Au and on Ag surfaces. The role of the substrate in the self-assembly
was investigated; for this purpose a novel ultra flat Au surface (Template
Stripped Gold) was developed. Different thicknesses of the organic
adlayer (length of the alkyl chain) and compositions of the adsorbate
(saturated or unsaturated chain) have shown distinct electric properties
of the molecular adlayer.

In the latter case, using intramolecular, intermolecular
and interfacial forces highly ordered 2D and 3D polymolecular micro-
and nano-scopic architectures from rod- and disc-like conjugated molecules
were produced. STM investigations at the interface between an almost
saturated solution and a graphite (HOPG) substrate allowed to characterize
both the structure and the dynamics of the molecular adsorbates. Phenyleneethynylene
trimers pack in an oriented 2D polycrystalline structure. The dynamics
of the single nanorods on a several minutes time scale was recorded.
This Ostwald ripening phenomenon is driven by a minimization of the
line energies. Such a high resolution imaging enabled us to gain insight
into the kinetics of this process and to draw conclusions on thermodynamic
and kinetic contributions to the total energy governing this grain
coarsening. The corresponding polydisperse system (PPE) is the first
polymer which was visualised with a sub-molecular resolution at the
solid-liquid interface. The macromolecules arrange into a nematic-like
molecular order on HOPG. Single rods are oriented along preferential
directions according to the symmetry of the substrate. The true molecular
lengths for several hundreds of physisorbed molecules were determined
from STM images. The key result was a narrow macromolecular fractionation
occurring at the solid-liquid interface: molecules belonging to the
two tails of the molecular weight distribution, namely shorter and
longer rods, do not favourably pack on HOPG. This result, which was
confirmed by Montecarlo calculations, could be interpreted in terms
of the thermodynamics of the physisorption at the solid-liquid interface
and in particular with respect to the enthalpic and entropic contributions
to the total free energy governing this process.

In addition, dried macromolecular films of PPE were
self-assembled by solution casting on insulating substrates under
various boundary conditions and studied with Tapping Mode - SFM (TM-SFM).
It was possible to understand and drive the growth of these architectures
towards epitaxially oriented micrometer long nanoribbons. These nanostructures
are typically two monolayers thick with their alkyl side-chains oriented
perpendicular to the substrate. The distribution of ribbon widths
is in good agreement with the molecular weight distribution according
to the Schulz-Zimm distribution. This indicates that SFM offers a
valuable alternative route to determine molecular weight distributions
for rod-like polymers. Moreover, these nanoribbons are architectures
from p-conjugated molecules which upon
thiol functionalization at their edges are ready to bridge Au nanoelectrodes
in a molecular nanowire device (Figure 1).

Figure
1: Cartoon of the approach that can be undertaken for developing a
hybrid metallic-molecular nanowire: This strategy allows to measure
the electric properties of the molecular nanowire.

Results on the electric properties of PPE aggregates
between the two Au nanoelectrodes are also presented. Moreover the
electronic structures of thin films of pristine and n-doped phenyleneethynylenes
were investigated with photoelectron spectroscopies corroborated by
theoretical calculations.

Furthermore highly ordered layers of synthetic disc-like
"nano-graphene" molecules, namely hexakis-dodecyl-hexabenzocoronenes
(HBC)s, were grown from solutions. TM-SFM and angle-resolved photoemission
measurements revealed that HBCs can self-assemble on conductive HOPG
substrates into monolayers with the p-conjugated
core lying preferentially parallel to the basal plane of HOPG. By
tuning the rate of the self-assembly, at very slow deposition speeds,
it was possible to produce layers aligned preferentially along the
crystallographic axes of HOPG. This suggests that the growth of HBC
is a kinetically governed phenomenon which on the crystalline support
in equilibrium allows a hetero-epitaxial type of growth.

In summary, making use of intra-molecular, inter-molecular
and interfacial forces it is possible to grow highly ordered conjugated
(macro)molecular nanostructures, which are candidates to be interfaced
to metallic nanojunctions to probe the conductivity of this molecular
ensemble.